#### [SOLVED] How to filter a list based on elements in another list in python

By Kingle

I have a list A of about 62,000 numbers, and another list B of about 370,000. I would like to filter B so that it contains only elements from A. I tried something like this:

``````A=[0,3,5,73,88,43,2,1]
B=[0,5,10,42,43,56,83,88,892,1089,3165]
C=[item for item in A if item in set(B)]
``````

Which works, but is obviously very slow for such large lists because (I think?) the search continues through the entire B, even when the element has already been found in B. So the script is going through a list of 370,000 elements 62,000 times.

The elements in A and B are unique (B contains a list of unique values between 0 and 700,000 and A contains a unique subset of those) so once A[i] is found in B, the search can stop. The values are also in ascending order, if that means anything.

Is there some way to do this more quickly?

#### @Fukiyel 2019-03-14 20:11:30

To be really sure what I've done is the fastest possible, I would have done that :

``````A=[0,3,5,73,88,43,2,1]
B=[0,5,10,42,43,56,83,88,892,1089,3165]

B_filter = B.copy()
C = []
for item in A:
if filter in B_filter:
C.append(item)
B_filter.pop(0) # B_filter is a list, and it's in ascending order so always the first
``````

If you don't care about losing your `B` list, you can just use `B` instead of `B_filter` and not declare `B_filter`, so you don't have to copy a 370k large list.

#### @Patrick Haugh 2019-03-14 20:07:49

This is creating a new `set(B)` for every item in A. Instead, use the built-in `set.intersection`:

``````C = set(A).intersection(B)
``````

#### @Christian Dean 2019-03-14 20:10:37

Nice! +1. What about `set(a)` & `set(B)`? Just seems a bit cleaner.

#### @Patrick Haugh 2019-03-14 20:11:55

@ChristianDean That requires a third set to be created, `set(B)`. In the above, only `set(A)` and `C` are created as new objects, `B` is just iterated over.

#### @Christian Dean 2019-03-14 20:12:43

Ah, I missed that. Good point.

#### @Kingle 2019-03-14 20:17:20

This is extremely fast (and bonus points for simplicity). Exactly what I was looking for. Thank you.

### [SOLVED] How do I get the number of elements in a list in Python?

• 2009-11-11 00:30:54
• y2k
• 3070342 View
• 1775 Score
• Tags:   python list

### [SOLVED] How do I remove an element from a list by index in Python?

• 2009-03-09 18:16:11
• Joan Venge
• 2193671 View
• 1230 Score
• Tags:   python list

### [SOLVED] How do I check if a list is empty?

• 2008-09-10 06:20:11
• Ray Vega
• 2218085 View
• 3237 Score
• Tags:   python list

### [SOLVED] How do I list all files of a directory?

• 2010-07-08 19:31:22
• duhhunjonn
• 3219309 View
• 3475 Score
• Tags:   python directory

### [SOLVED] Finding the index of an item given a list containing it in Python

• 2008-10-07 01:39:38
• Eugene M
• 3244821 View
• 2674 Score
• Tags:   python list indexing